Field of the Invention
The invention concerns a method, a magnetic resonance apparatus, and an electronically-readable data medium for correction of magnetic resonance image data.
Description of the Prior Art
Magnetic resonance (MR) technology is a known modality with which images of the inside of an object under examination can be created. Expressed in simple terms this involves positioning an object to be examined in a magnetic resonance scanner in a strong static, homogeneous basic magnetic field, also called the B0 field, with field strengths from 0.2 Tesla to 7 Tesla and more, so that nuclear spins in the object are oriented along the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency pulses (RF pulses), for excitation or refocusing, are radiated into the object under examination, and the nuclear magnetic resonance signals that are triggered are measured as so-called k space data. On the basis of the k-space data, MR images are reconstructed or spectroscopy data are established. For spatial encoding of the measurement data, rapidly switched magnetic gradient fields are overlaid on the basic magnetic field. The recorded measurement data are digitized and stored as complex numerical values in a k space matrix. From the k space matrix occupied by values, an associated MR image is able to be reconstructed, by a multi-dimensional Fourier transformation, for example.
All these technical subsystems, e.g. for gradient control and for RF send/receive control, must be accessed in a coordinated way by a control computer. The settings and switchings of the individual subsystems necessary for a specific measurement process must be undertaken at the correct point in time by the activation. Usually the volume to be mapped is recorded, e.g. within an imaging sequence, in sub-volumes, for example with 2D imaging in a number of slices or with 3D imaging in a number of so-called slabs. The sub-volumes thus recorded are then combined into a complete volume. A further definition of sub-volumes can be designated, for example, as Regions of Interest (ROI) or Volumes of Interest (VOI) that are defined specifically by the operator. Furthermore, additional sub-volumes are produced such as in magnetic resonance systems during the definition of local saturation regions or local preparation or labeling pulses.
For this purpose of coordinated activation, sequence control data, primarily based on a measurement protocol, can be transferred to the control computer. This sequence control data define various functional sub-sequences of a complete measurement sequence. For magnetic resonance imaging a first sub-sequence can involve a pulse sequence for example, which locally in a specific area achieves a saturation of specific spins. Further sub-sequences can contain specific preparation pulses, for example, and other sub-sequences are used for successive excitation and for receiving the magnetic resonance signals in different slices or slabs.
In general, methods based on magnetic resonance, especially tomographic imaging (MRT, Magnetic Resonance Tomography) and spectroscopy (MRS, Magnetic Resonance Spectroscopy) need “favorable” physical environmental conditions in order to ensure the best possible quality of the recorded data. For example, this involves at least one of the criteria of spatial homogeneity, temporal stability and absolute accuracy of the magnetic fields B0 (the stationary main magnetic field) and B1 (the magnetic radio-frequency alternating field) relevant for the MR method.
Known measures, with which deviations from ideal environmental conditions can be at least partly compensated, include system-specific settings that seek to correct the circumstances of the MR system used, such as e.g. eddy-current-induced dynamic field disturbances or gradient sensitivities, as well as examination object-specific settings, which attempt to balance out the changes caused by the introduction of the object under examination, e.g. a patient, into the measurement volume of the MR system, such as susceptibility-related static field disturbances or spatial variations of the radio-frequency field.
These types of methods for improving the quality of the recorded measurement data, especially by adjustments, and for dynamic adjustment, of the corresponding measurement parameters, have been further developed in recent years.
A method is described in DE 10 2009 020 661 B4, for example, with which parameters of a measurement sequence, e.g. in the magnetic resonance technology, can be adapted while the measurement sequence is running. In addition it is already described therein that different functional sub-sequences are generally assigned to different effective volumes. This means that for each sub-sequence, a different sub-volume of the overall measurement volume is relevant.
An adaptation of measurement parameters during an ongoing measurement for optimizing the image quality is also described in that document. The basic idea of such a dynamic adjustment is to arrange the physical environmental conditions where possible at each point in time such that they are as ideal as possible for the sub-volume currently relevant in the measurement process. If, for example, during an MR measurement the spins of a slice are excited and thereafter the created signal is detected, then for this time segment of the MR measurement the measurement parameters can be optimized to the region defined by the slice. During a following excitation and detection of the next slice the optimization can then be accordingly dynamically adapted, etc.
The measurement parameters to be adapted can include the mid frequency in the modulation of the radiated RF pulses, the demodulation frequency of the received MR signal, scaling factors of the RF pulse amplitude, amplitude and phase distribution of the RF currents to a number of send elements (where present), B0 shim settings (of first or higher order for example), transmitter scalings, B1 shim settings or also Maxwell compensation settings. As a result of the local environmental conditions optimized at any given point in time of the measurement, the image quality is prospectively significantly improved by such dynamic adjustments—even by comparison with static adjustment settings.
Furthermore MR recording and post-processing methods are known that, on the basis of environmental condition maps established in advance of a diagnostic measurement, make possible a correction of MR images, such as a retrospective correction. The environmental condition maps provide knowledge about the environmental conditions, e.g. in the form of spatially-resolved maps of e.g. the actual field distribution of the basic magnetic field B0 and/or of the radio-frequency alternating field B1. Such methods include, for example, methods for correction of image distortions resulting from basic field inhomogeneities, methods for correcting the influence of Maxwell fields, methods for correction of parameter maps, methods for correction of the influence of gradient non-linearities or also methods for computing optimized (e.g. multi-dimensional) RF pulses. These types of correction methods are frequently needed for example for correction of distortions and other artifacts.
These types of correction method include a correction of susceptibility-related distortions in echo planar imaging (EPI), for example.
EPI methods typically exhibit a very small bandwidth of the pixels in the phase encoding direction (e.g. a few 10 Hz/pixel). Therefore, the mapping fidelity in EPI methods is especially sensitive to (local) variations of the basic magnet field B0. These types of variations (inhomogeneities) can be induced for example by susceptibility differences of different tissue types as well as by the surrounding air.
A number of methods for correction of these types of mapping errors operates on the basis of recorded B0 field maps. As an example, reference can be made to the article by Jezzard et al. “Sources of Distortion in Functional MRI Data”, Human Brain Mapping 8, P. 80-85 (1999). These types of field maps are recorded before or during the measurement of the measurement data. Knowledge of the B0 field distribution from the B0 field map successfully enables image distortions to be at least partly reversed by suitable processing steps (see e.g. the said article by Jezzard et al.). Basically similar correction methods are not only applicable for EPI methods, but for all imaging methods that exhibit a high B0 sensitivity, such as e.g. also gradient echo imaging (GRE) with low bandwidth, measurement data acquisition methods with spiral trajectories, etc.
Other methods for correction of these types of mapping error are used for the correction of distortions or of undesired phase errors as a result of Maxwell field terms. These types of distortions or errors arise e.g. during switching of magnetic field gradients for MR imaging, since here, as well as the desired longitudinal components in accordance with the Maxwell equations, undesired (but entirely MR-relevant) cross components also occur. The latter can lead, for example, to distortions in the echo planar imaging, or to additional phase evolutions, which can cause an undesired signal dephasing in diffusion or flow imaging. This problem is described, for example, in the article by Meier et al. “Concomitant Field Terms for Asymmetric Gradient Coils: Consequences for Diffusion, Flow, and Echo-Planar Imaging”, Magnetic Resonance in Medicine 60, P. 128-134 (2008).
Methods for correction of these types of Maxwell effects usually operate on the basis of the known relationship between longitudinal magnetic useful field and the associated field deviations, as is described, for example, in the article by Du et al. “Correction of Concomitant Magnetic Field-Induced Image Artifacts in Nonaxial Echo-Planar Imaging”, Magnetic Resonance in Medicine 48, P. 509-515 (2002). In this way a map of the effects to be expected can be created for the respective effect of the switched gradient pulses, in order to carry out compensating measures (for example removing distortion from the images) on this basis.
Yet other methods for correction of these types of mapping errors are used to compute RF pulses for localized (e.g. two- or three-dimensional) excitation. These types of specific RF pulses, which are applied simultaneously with an adapted gradient trajectory for example, allow a dedicated excitation of “shaped” areas in the object under examination. In this way, for example, only the desired examination areas can be recorded explicitly or explicitly undesired areas in the object under examination (e.g. parts that move, which can lead to image artifacts) can be saturated and thus suppressed in the final image.
For computing the shape and nature of such RF pulses (on one or more transmitter channels) as well as of the associated gradient pulses, B0 and B1 maps are indispensable as a rule. An example for such a computation is described in the article by Setsompop et al. “Slice-Selective RF Pulses for In Vivo B1+ Inhomogeneity Mitigation at 7 Tesla Using Parallel RF Excitation With a 16-Element Coil”, Magnetic Resonance in Medicine 60, P. 1422-1432 (2008). These B0 and B1 maps can be recorded before or during the measurement.
Further methods for correction of these types of mapping error can be used for correction of parameter maps. For example in quantitative MR imaging, instead of grayscale images with undefined scaling, spatially resolved parameter maps (e.g. for T1, T2, T2*) with defined scaling are generated. In a few of the associated recording and computing methods variations in specific environmental parameters lead to errors in the quantification. This is especially the case for variations of the amplitude of the local B1 field of the local flip angle produced thereby.
A number of correction methods for quantitative MR imaging takes into account the information from recorded field maps (e.g. of the B1 field) for rectifying the quantification errors. An example for such a correction method is described in the article by Cheng et al. “Rapid High-Resolution T1 Mapping by Variable Flip Angles: Accurate and Precise Measurements in the Presence of Radiofrequency Field Inhomogeneity”, Magnetic Resonance in Medicine 55, P. 566-574 (2006).
Other methods for correction of such mapping errors are used for correction of distortions resulting from non-linearities of the gradient fields. This is because the gradient fields used for the spatial assignment in MR imaging generally (for practical reasons) at least in the edge area of the mapping volume, exhibit deviations from a perfectly linear curve. As a result images in these areas exhibit distortions.
Methods for correction of such distortions generally operate on the basis of the known spatial geometry of the gradient fields. This information is used during image processing in order to assign the recorded data to a corrected spatial position. Such a correction method is described for example in U.S. Pat. No. 4,591,789A1.
All these correction methods however, when applied to MR images that have been created using measurement data acquired from the aforementioned adjustment methods, lead to errors, since the underlying maps under some circumstances are no longer correct as a result of the dynamic adjustments.